This application claims the priority of German Application No. 196 30 209.9-52, filed Jul. 26, 1996, the disclosure of which is expressly incorporated by reference herein.
The invention relates to the use of a gas sensor for the selective detection of hydrocarbons (in the following also abbreviated HC) in low-oxygen gases.
It is known to use hydrocarbon sensors to determine the efficiency of exhaust gas after treatment systems on motor vehicles having Otto internal-combustion engines. A special requirement with respect to the hydrocarbon sensor is its insensitivity to CO and H.sub.2, since both such constituents are present in large quantities--compared with hydrocarbons--in the exhaust gas of Otto engines.
European Patent Document EP 0 426 989 B1 discloses a selective chemical sensor for gases which meets these requirements. In one embodiment, the sensor consists of a platinum-containing zeolitic layer on an interdigitated capacitor structure. (In the following, "interdigitated capacitor" is sometimes abbreviated IDC). In "Sensors and Actuators", B 24-25 (1995) 403-406, it is shown that a PTNaY-IDC sensor produced on this basis by means of screen printing at 350.degree. on air has 1,000 ppm butane while, at the same time, neither 1,000 ppm CO, nor 1,000 ppm H.sub.2 are detected.
Another difficult requirement when using Otto internal-combustion engine is the considerably reduced oxygen concentration (in the range of .lambda.=1) in comparison to atmospheric conditions. Simultaneously, hydrocarbons in concentrations of significantly below 100 ppm must be detected.
The sensors described in the two above-mentioned documents relate to the detection of hydrocarbons exclusively in high-oxygen gases, for example, in ambient air (oxygen proportion approximately 20%). In the case of a high oxygen concentration, the hydrocarbons and the CO and the hydrogen in the course of a catalytic total oxidation are in each case completely converted into the thermodynamically most stable compounds. In the case of hydrocarbons, CO.sub.2 and H.sub.2 O are produced; in the case of CO, only CO.sub.2 is produced; and, in the case of H.sub.2, only H.sub.2 O is produced. The catalytic activity of the precious-metal-containing zeolites considered here is so high that H.sub.2 is completely converted even at room temperature; and CO is completely converted at a temperature below 100.degree. C. In contrast, a significantly higher temperature is required for the catalytic conversion of hydrocarbons. At the operating temperature of the hydrocarbon sensor, the reaction duration for the total conversion of hydrocarbons is significantly greater than that of H.sub.2 and CO.
At a given temperature, cations jump to adjacent adsorptive states in zeolite's pore system under the force of an electric field so that a mobility of cations in the zeolite pore system is established. In case of an alternating voltage and given cation mass, mobility depends on frequency. Thus, the zeolite sensor system will respond with frequency dependent impedance when alternating voltage is applied. The cation mobility may be influenced, if a chemical reaction takes place within the extremely narrow pore systems of zeolites. Whether or not a measurable sensor effect occurs depends qualitatively on whether or not the duration of catalytic reaction and cation jump velocity will fit together.
The duration of the catalytic reaction, the ionic conductibility of the zeolite at the operating temperature of the sensor, the steric conditions of the pore system of the zeolites (that is, the spacing of the cationic adsorption sites and of the catalytically effective centers) must be precisely coordinated with one another. In the case of catalytical total oxidation of hydrocarbons the mobility decreases which leads to the sensor effect in EP 0 426 989 B1. But in the case of catalytic total oxidation of hydrogen and CO at comparable temperatures no sensor effect occurs, as the duration of catalytic reaction and cation jump velocity don't fit.
In the case of very low oxygen concentrations, the hydrocarbons are only partially oxidized instead of totally oxidized as described above, and different reaction times and reaction products will occur. For such catalytical partial oxidation of hydrocarbons which is the subject of the present patent it is unknown whether or not duration of catalytic reaction and cation jump velocity will fit together. No person skilled in the art therefore could say in advance whether a sensor effect will occur with this type of sensor in the case of catalytical partial oxidation of hydrocarbons. That is, the essential principle which the function of the sensor according to the EP 0 426 989 B1 is based on is a total oxidation. Consequently a person skilled in the art should expect, that this sensor doesn't work when a gas containing a low concentration of oxygen is used. Thus, the above-mentioned known hydrocarbon sensors do not appear to be suitable for detecting hydrocarbons with very low oxygen contents, such as occur in the exhaust case of Otto internal-combustion engines wherein .lambda.=1.
One object of the invention is to provide a sensor for the selective detection of hydrocarbons in low-oxygen gases (O.sub.2 -proportion lower than 10,000 ppm).
This and other objects and advantages are achieved by the hydrocarbon detector according to the invention. Surprisingly, it was found that the hydrocarbon-sensitive sensor, which is known per se from European Patent Document EP 0 426 989 B1, also has a high sensitivity in the case of a reduced oxygen content. The sensor according to the invention therefore comprises:
a component operating as a capacitor, PA1 a gas-permeable sensitive layer as a dielectric, the sensitive layer being a precious-metal-doped zeolite which has a regular crystalline structure made of primary pores whose diameter is on the order of the gas-kinetic diameter of the gas molecules to be detected.
The sensor is based on the following measuring principle: The impedance of the sensor is (as noted previously) a function of both the applied frequency and the ionic conductibility of the zeolite, the latter being influenced by the interaction of the measuring gas component with the interior zeolite surface. The catalytic conversion of the hydrocarbons on the precious-metal centers in the pores of the zeolites, at the sensor operating temperature, does not occur infinitely fast, but at a finite speed. During the time period of the catalytic conversion, the mobility of the cations in the electric field is hindered by the reaction which results in an increase of the resistance in the considered frequency range. The operating temperature of the sensor is in the range between 300.degree. C. and 500.degree. C.
Detection is achieved by determining the impedance of the sensor at a suitable fixed frequency.
In a preferred embodiment, a ZSM5-zeolite is used as the zeolite, particularly having a module SiO.sub.2 /Al.sub.2 O.sub.3 (that is, a ratio of the content of SiO.sub.2 and Al.sub.2 O.sub.3 to zeolite) of 20 to 50. Pt or Pd, particularly at a proportion of from 0.1 to 5% by weight are preferably used as zeolite crystallites. A ZSM5-zeolite with a Pt fraction of 3% by weight in the zeolite powder was found to be particularly advantageous.
The following table lists several zeolites according to the invention together with their respective pore diameter:
______________________________________ ZSM5 0.53 nm * 0.56 nm mordenite 0.67 nm * 0.70 nm beta 0.76 nm * 0.64 nm Y 0.74 nm ______________________________________
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.